1. Field of the Invention
The invention relates to extrusion dies for extrusion of plastic tube having a smooth inner surface and an annularly ribbed or finned outer surface, i.e. an outer surface having alternating annular ribs and grooves.
2. Background of the Invention
Plastic tube for a variety of purposes, for example, underground drain pipes frequently needs considerable strength against crushing. Often such pipe is corrugated for strength or is double walled corrugated pipe. The manufacture of corrugated single or double walled pipe has necessitated the design of a variety of apparatus to produce suitable pipe. Double walled pipe especially, has provided some problems.
It has become apparent that, at least for some uses, ribbed pipe rather than corrugated pipe has certain advantages. Ribbed pipe may have good stiffness against crushing and its manufacture was thought to be free of the difficulties associated with uniting the two walls of double walled corrugated pipe. Ribbed pipe may be manufactured by extruding a continuous parison into a travelling mold of mold blocks defining the ribs and grooves of the resulting pipe. Examples of such apparatus are disclosed in U.S. Pat. No. 3,891,007 issued June 1975 to Kleykamp, U.S. Pat. No. 4,710,337 issued January 1987 to Nordstrom, and U.S. Pat. No. 4,721,594 issued January, 1988 to Jarvenkyla.
At least Kleykamp, in U.S. Pat. No. 3,891,227, appears to have appreciated that difficulty might be encountered in filling the mold cavities in that he fills the mold cavities with one stream of extrudate and then applies a smooth inner wall. The ribs used by Kleykamp do not appear very deep in comparison with the diameter of the resulting tube and are referred to as "an undulating outside surface". Nordstrom in U.S. Pat. No. 4,710,337 also clearly appreciates the problem and solves it in a similar manner to Kleykamp i.e. he first fills the ribs and secondly thickens the tubing wall.
In fact, in practice, difficulties are encountered in filling the mold cavities to form the ribs.
Jarvenkyla in U.S. Pat. No. 4,721,594 illustrates exit angles of extrudate of less than 45.degree. and, alleges that an angle of 2 to 30.degree. is important. Moreover, Jarvenkyla discusses the pressure variations set up and attempts to justify some correlation with an apparent angle of between 2 and 30.degree. to the axis.
When a mold cavity of a travelling mold for forming a rib comes into a position to be filled from an extrusion orifice of an extrusion nozzle, the cavity to be filled is appreciable and the extrudate flows easily resulting in a reduction of pressure in the nozzle. As the mold advances, a ridge of the travelling mold comes adjacent the extrusion orifice causing pressure to rise. As a result, a rhythmic pressure surge may be set up producing various apparatus strains and resulting wear.